1. Field of the Invention
The present invention relates to a boundary acoustic wave filter device utilizing a boundary acoustic wave propagating along a boundary surface between a piezoelectric substrate and a dielectric film, and more particularly, the present invention relates to a boundary acoustic wave filter device having a circuit configuration in which a one-terminal-pair boundary acoustic wave resonator is connected to a longitudinally coupled resonator boundary acoustic wave filter.
2. Description of the Related Art
Recently, surface acoustic wave filter devices have been used as bandpass filters in RF stages of mobile phones because of their smaller size and excellent filter characteristics.
For example, in Japanese Unexamined Patent Application Publication No. 2002-64358, a compound surface acoustic wave filter device in which a one-port-type surface acoustic wave resonator is connected in series to a longitudinally coupled resonator double-mode surface acoustic wave filter is disclosed.
FIG. 5 is a schematic plan view of the schematic configuration of this type of compound surface acoustic wave filter device. A compound surface acoustic wave filter device 501 implements an electrode configuration shown in the drawing on a piezoelectric substrate 502. That is, IDTs 504 to 506 are arranged in the propagation direction of a surface-acoustic wave. Reflectors 507 and 508 are each disposed on a corresponding side of a region in which the IDTs 504 to 506 are provided, in the propagation direction of the surface wave, whereby a longitudinally coupled resonator double-mode surface acoustic wave filter 503 is configured. One end of the IDT 505 is connected to an input terminal 511. The IDTs 504 and 506, each of which is disposed on a corresponding side, are commonly connected to each other and connected to a one-terminal-pair surface acoustic wave resonator 509. The one-terminal-pair surface acoustic wave resonator 509 is connected to an output terminal 512.
In the surface acoustic wave filter device 501, the one-terminal-pair surface acoustic wave resonator 509 is connected in series to the longitudinally coupled resonator double-mode surface acoustic wave filter 503, thereby increasing an out-of-band attenuation.
Furthermore, considering that it is difficult to obtain a sufficient attenuation by using only the above-described configuration, a capacitor 513 is connected to a path that connects the longitudinally coupled resonator double-mode surface acoustic wave filter 503 and the one-terminal-pair surface acoustic wave resonator 509 in series. That is, a path including the capacitor 513 serves as a bypass path, and signals in a stopband located in a higher frequency range than a passband flow to the ground potential. This improves attenuation in the high frequency range located outside of the passband.
In contrast, in Japanese Unexamined Patent Application Publication No. 7-131290 described below, a configuration with a parallel-connected resonant element instead of the capacitor 513 is disclosed. In a case where a resonator is used, a steep impedance change at a resonant frequency can be obtained. Accordingly, even when a low impedance peak is located outside of a passband in a low frequency range, impedance can be maintained to be high in the passband. As a result, attenuation in the low frequency range located outside of the passband can be sufficiently increased without causing any large effects on the passband. In Japanese Unexamined Patent Application Publication No. 7-131290, as described above, a resonant frequency of the parallel-connected resonator is located in a stopband that is located outside of the passband of the filter in the low frequency range, and an anti-resonant frequency of a serially connected resonator is located in a higher frequency range than the passband. That is, with this configuration, the parallel-connected resonator, which is used instead of the capacitor 513, improves the attenuation in the low frequency range located outside of the passband.
In the surface acoustic wave filter device 501, which is disclosed in Japanese Unexamined Patent Application Publication No. 2002-64358, as described above, the serially connected type one-terminal-pair surface acoustic wave resonator 509 is connected in series to the longitudinally coupled resonator double-mode surface acoustic wave filter 503, and the capacitor 513 is also connected in parallel to the serial path. This allows an out-of-band attenuation, in particular, attenuation in the high frequency range located outside of the passband to be increased.
In order to improve the attenuation in a stopband using a capacitive element such as the capacitor 513, it is necessary that the capacitive element has a low impedance to signals at frequencies in the attenuation in a stop band and a high impedance to signals at frequencies in a passband.
However, merely using the capacitor 513 does not cause a steep change in impedance. For this reason, when impedance in the stopband is set to be low in order to sufficiently increase attenuation in the vicinity of the passband, there is a disadvantage in that impedance in the passband does not become sufficiently high. In such a case, some of the signals in the passband flow to the capacitor 513, resulting in a deterioration in the characteristics of the passband. Conversely, when the impedance to the signals in the passband is set to be sufficiently high, there is a disadvantage in that the impedance in the stopband does not become sufficiently low, resulting in the attenuation in the stopband not being sufficiently increased.
In the surface acoustic wave filter device disclosed in Japanese Unexamined Patent Application Publication No. 7-131290, instead of the capacitor 513, the parallel-connected surface acoustic wave resonator is connected to the path. In this case, even when a resonance point, which is a low impedance peak, is located outside of the passband in the low frequency range, high impedance can be achieved in the passband. Accordingly, the attenuation in the low frequency range located outside of the passband can be increased without causing any large effects on the characteristics of the passband.
In the configuration disclosed in Japanese Unexamined Patent Application Publication No. 7-131290, the resonant frequency of the parallel-connected surface acoustic wave resonator is located in the stopband that is located outside of the passband of the filter in the low frequency range, and an anti-resonance point of the serially connected surface acoustic wave resonator is located outside of the passband in the high frequency range, whereby the parallel-connected-type surface acoustic wave resonator increases the attenuation in the low frequency range located outside of the passband. In order to increase attenuation in the high frequency range located outside of a passband, a resonance point of the parallel-connected surface acoustic wave resonator can be located in a higher frequency range than the passband of a filter. In this case, the impedance of the parallel-connected surface acoustic wave resonator is capacitive in the passband.
The larger the electromechanical coefficient of a surface acoustic wave filter, the less the capacitive IDTs with which the surface acoustic wave filter is configured seem to be. Accordingly, when the electromechanical coefficient is comparatively small, the IDTs seem to be capacitive. Thus, because all of the filter, the serially connected surface acoustic wave resonator, and the parallel-connected surface acoustic wave resonator seem to be capacitive, there is a disadvantage in that it is difficult to achieve impedance matching in the passband.
The electromechanical coefficient of the IDTs is represented by K2, a capacity ratio of the IDTs is represented by γ. Because the equations γ=C0m/Cm (C0m represents parallel capacitance when the IDTs are represented by an equivalent circuit, and Cm represents a capacitive element in a series-resonant circuit that represents a function of emitting a surface-acoustic wave) and K2=1/(1+γ) are obtained, the relationship γ=(1/K2)−1 can be achieved.
That is, the larger the electromechanical coefficient K2, the smaller the capacity ratio γ. That is, C0m/Cm becomes small. In other words, when the electromechanical coefficient K2 is small, the admittance of C0m seems to be large with respect to the admittance of the series circuit that emits a surface-acoustic wave. As a result, the IDTs in the surface acoustic wave filter seem to be capacitive. Conversely, when the electromechanical coefficient K2 is large, the IDTs seem to be less conductive.
In the related art, in the surface acoustic wave filter device, because the electromechanical coefficient is comparatively small, the IDTs in the surface acoustic wave filter seem to be capacitive. In a case where the serially connected surface acoustic wave resonator and the parallel-connected surface acoustic wave resonator are connected to the surface acoustic wave filter and a resonance point of the parallel-connected surface acoustic wave resonator is located outside of the passband in a high frequency range, in addition to that, the original impedance is capacitive, the capacitive parallel-connected-type surface acoustic wave resonator is connected to the surface acoustic wave filter. Accordingly, there has been a disadvantage in that it is difficult to achieve impedance matching in the passband.
Thus, in the surface acoustic wave filter device in the related art, the parallel-connected surface acoustic wave resonator, which is connected in parallel to the surface acoustic wave filter, has not been used to improve the attenuation in the high frequency range located outside of the passband.
Recently, instead of a surface acoustic wave filter device, a boundary acoustic wave filter device in which a wave propagates along a boundary between different media has drawn attention. Because in a boundary acoustic wave filter device, no space needs to be provided, a packaging configuration can be more simplified and miniaturized. Furthermore, as in the case of a surface acoustic wave filter device, the improvement of an out-of-band attenuation has been strongly requested also in a boundary acoustic wave filter device.